I was always under the impression that the lower you go the harder it is to hear distortion. Apparently a new range of subs is out which states otherwise. My thoughts are if you hear distortion at those frequencies you would hear "more" further up. Any thoughts on this?

Low end distortion will cause harmonic distortion further up the range. The degree depends on the driver, but push any driver hard enough and you will eventually begin to notice. Push pull helps reduce this, but does not completely remove it. The benefits though are only really going to be of benefit if you thrash your sub, and thats something I have tested myself. Unless I thrashed my PP design, the PP mechanic seemed to make no difference to audible sound quality. Perhaps if your going to use smaller more average drivers in a fairly compact cabinet, push pull is worth the difficulty in its implementation, otherwise I would say its not something you really need to be worrying about, unless your pushing your dual driver sub to the limit anyway.

Nothing negates the PP design, if you want to take advantage of the benefits of it. Every driver distorts to varying degrees at varying levels, and the PP mechanic will help reduce the harmonic distortion present in any driver. The question really is, will you be pushing your sub hard enough for it to matter. Its like adding something to you car that will improve its handling over 150 mph, if you never hit those speeds, then is there really any benefit. For me personally, I used larger drivers, and employing push pull becomes quite a headache to put together when your trying to keep already larger cabinet sizes down, so I opt not to go PP. I did experiment with it to be sure though.

Shorting rings in a driver help to keep it cooler, so they are something desirable in a driver design, though it adds to cost and is something you generally see in more expensive models.

I was always under the impression that the lower you go the harder it is to hear distortion. Apparently a new range of subs is out which states otherwise. My thoughts are if you hear distortion at those frequencies you would hear "more" further up. Any thoughts on this?

Our hearing is less sensitive to low frequencies, low frequency distortion primarily consists of harmonics of the fundamental frequencies, where our ears are more sensitive.
For instance, the second harmonic of 30Hz is 60 Hz, 60Hz at 75 dB would sound as loud as 30 Hz at 90 dB. A harmonic only 15 dB down from the fundamental is about 18% distortion, when a sub is driven below Fb distortion can rise to 100% quite rapidly.

Push pull reduces even order harmonics, leaving odd order harmonics.
Even order harmonics are musical sounding, octaves of the fundamental.
Odd order harmonics often sound bad, as they change the nature of the music.

Reduction of even order harmonics while leaving odd order harmonics may subjectively sound worse, as I found out in this test: